Controlling Molecular Vibrational Quantum State And Field-free Orientation By Ultra-short Pulse Laser

The works in this paper are firstly based on the time-dependent quantum wave packet method, to investigate theoretically the population transition of diatomic molecules by ultra-short pulse laser. Second, using a graphical method, we derived the density matrix equations of Stark-induced adiabatic Raman passage describing population transition. Finally, we investigate the field-free orientation using a single-cycle terahertz pulse. The main works are as follows.(1) The isotope effects of XF (X=H, D) on the population transfer process via two-photon resonance excitation are investigated by solving the time-dependent Schrodinger equation. The vibrational levels v=0 and 2 of the ground electronic state are taken to be the initial and target states, respectively, for the two molecular systems. The influences of the field peak amplitude and pulse duration to population transfer efficiency and isotope effects are discussed in detail. The pulse duration is required to be longer than 860 fs for the DF molecule to achieve relatively high transfer efficiency (more than 80%), while the one for the HF molecule is just required to be longer than 460 fs. Moreover, the intermediate level v=1 and the higher level ?=3 may play more important roles in the two-photon resonance process for the DF molecule, compared to the roles in the process for the HF molecule.(2) In the absence of a real intermediate level, the stimulated Raman transition v=0? v=1 driven by the coherent pump and Stokes fields is reduced to the problem of a two-photon resonant two-level system. Using a graphical method, we derived the density matrix equations of Stark-induced adiabatic Raman passage in the presence of polarized pump and Stokes optical fields. This method exploits the dynamic Stark shifts of rovibrational levels |v, J,M> with a delayed sequence of overlapping pump and Stokes pulses of unequal intensities.(3) The influence of the carrier envelope phase (CEP) on the field-free orientation of the Nal molecule is investigated theoretically. The molecular orientation steered by a single-cycle terahertz (THz) pulse is studied by solving exactly the time-dependent Schrodinger equation including the rovibrational degrees of freedom. We find that the field-free orientation is dependent on the central frequency and CEP of the single-cycle pulse. The CEP of single-cycle THz pulse affects the field-free orientation mainly via changing the population distributions of rotational states. By choosing the CEP of single-cycle THz pulse at the appropriate central frequency, the maximal value of the degree of field-free orientation, <cos ?>, can reach 0.80. Moreover, we find that the duration of effective molecular orientation (|<cos ?>|?0.5) is enough long in certain region of the laser central frequency and CEP.